1. Field of the Invention
The present invention relates to an optical modulating device, particularly to an optical modulating device with frequency multiplying technique for electrical signals.
2. Description of the Related Art
Fiber-optic communication is a communication method based on light and optical fibers. Light can be used to carry information after being modulated where it features a superior transmission capacity and a high information security. Since 1980s, the fiber-optic communication system plays a very important role in the digital world. In principle, the outgoing information is transferred from the sender side to the transmitter, and the signal modulates the carrier wave, which functions as the transmission medium of information, and then the modulated carrier wave is sent to the recipient side in a distant place, and the receiver demodulates the modulated carrier wave to obtain the original information.
System utilizing optical fiber to carry and transmit microwave signal would play an important role in future wideband wireless communication especially on millimeter-wave range of ultra wideband wireless communication that has frequency band above 40 GHz. The most common method for transforming microwave signal to optical signal is using an external optical modulator, and there are mainly three kinds of modulation methods: double-sideband (DSB), single-sideband (SSB) and double-sideband with optical carrier suppression (DSBCS) modulation. However, due to the fact that linear area for optical modulation system is not large, the depth of modulation for the system is being limited. As a result, using DSB and SSB for modulation will cause weaker sensitivity over distant transmission and dispersion of optical fiber will also weaken the microwave signal using DSB modulation. Compare to modulations using either DSB or SSB, the DSBCS modulation provides more promising bandwidth efficiency which requires only low frequency component, it also provides better sensitivity over long distance transmission. Nevertheless, DSBCS modulation can only transmit amplitude shift keying (ASK) modulation signal, it can not generate and transmit any signal in vector modulation format such as phase shift keying (PSK), quadrature amplitude modulation (QAM) signal and orthogonal frequency division multiplexing (OFDM) modulation signals.
To overcome the abovementioned problem, the present invention proposes a novel optical modulating device with frequency multiplying technique for electrical signals, not only does the present invention able to use low frequency component to generate frequency multiplying signals, but the present invention can also carry and transmit amplitude shift keying signals and vector modulation signals. In addition, the present invention can be combined with optical fiber cable network and wireless network to provide long distance optical communication transmission service.